Google Adwords now gives advertisers the choice of where in the Google content network they’d like their ads to appear. Google describes site targeting as:
Site targeting lets AdWords advertisers choose individual sites in the Google content network where they’d like their ads to appear. If you want to place your ad on a single international website, you can. If you want to select dozens of sites about basketball or orchids, you can do that, too. Site targeting lets you handpick your audience, big or small.
You can also choose site sections as described by Google:
Site targeting places your ads on individual sites in the Google content network. Site sections take that one step further by placing your ads on only one section or even one page of a site. If you sell soccer shoes, for instance, you might choose to advertise only on the sports section of a news site rather than placing ads across the entire site.
This gives you tremendous flexibility as a search engine marketer in having your ads show up only on sites that rank and only show on specific pages.